1. Field of the Invention
The invention relates to the attenuation of noise made in a turboengine by interaction between the blades of a rotor blade ring and the vanes of a stator vane ring disposed downstream of the rotor blade ring, wherein a counternoise of opposite phase to the noise is created by the injection of fluid through a number of orifices equal to the number of stator ring vanes.
2. Summary of the Prior Art
FR-A-2370170 specifies orifices in the flow duct downstream of the rotor blade ring, either before or aft of the vanes. The orifices are fixed relative to the stator but the fluid flow injected through each orifice must be modulated by a valve actuated at the frequency of the noise to be attenuated.
A modulating value must therefore be associated with each orifice and all the valves are controlled by an electronic servo control unit by a magneto-type electronic device to dephase the flow between the various orifices.
The system proposed by FR-A-2370170 to attenuate the noise arising from the interaction between the moving blades and the fixed vanes is therefore relatively complex and therefore very prone to failure.
It is an object of the invention to provide a simple process requiring few moving parts for attenuating noise arising from interaction between moving blades and fixed vanes disposed immediately downstream of the moving blades.
Accordingly, the invention proposes a process for attenuating noise generated by the interaction between a ring of moving rotor blades and a ring of fixed stator vanes in the flow channel of a turboengine, said fixed stator vanes being disposed downstream of said moving rotor blades, comprising the steps of providing a plurality of orifices, equal in number to the number of said fixed stator vanes, opening into said flow channel upstream of said ring of rotor blades, injecting a fluid into said flow channel through said orifices to form continuous jets of said fluid upstream of said ring of rotor blades, and adjusting the angular position of said orifices relative to said fixed stator vanes whereby sound waves generated by interaction between said jets of fluid and said moving rotor blades are of substantially opposite phase to sound waves generated by interaction between said moving rotor blades and said fixed stator vanes.
In the proposed process there is therefore no modulation of the jet deliveries, because the sound waves arising from interaction between the jets and the moving blades are at the same frequency as the sound waves arising from interaction between the moving blades and the fixed vanes.
In a first variant of the process the angular position of the orifices relative to the vanes is adjusted continuously.
In a second variant of the process the angular position of the orifices relative to the vanes is adjusted intermittently in dependence upon engine speed.
When the process according to the invention is used to attenuate the noise made by a fan of a bypass turbojet engine whose flow duct has a pylon or arms extending through it, the position and diameter of the injection orifices are so calculated as to take into account the velocity distortion caused by the pylon and the arms.
It is further object of the invention to provide a device for carrying the process into effect, and to this end the invention provides a device for attenuating noise generated by the interaction between a ring of moving rotor blades and a ring of fixed stator vanes in the flow channel of a turboengine, said fixed stator vanes being downstream of said moving rotor blades, comprising a ring mounted on the engine casing upstream of said ring of rotor blades and rotatable about the rotational axis of said ring of rotor blades through an angle at least equal to the angular pitch of the fixed stator vanes; a plurality of orifices, equal in number to the number of said fixed stator vanes, providing in said ring and opening into said flow channel upstream of said ring of rotor blades, means for injecting a fluid into said flow channel through said orifices to form continuous jets of said fluid upstream of said ring of rotor blades, and means for rotating said ring to adjust the angular position of said orifices relative to said fixed stator vanes whereby sound waves generated by interaction between said jets of fluid and said moving rotor blades are of substantially opposite phase to sound waves generated by interaction between said moving rotor blades and said fixed stator vanes.
To carry into effect the first variant of the process, the device also comprises a motor which drives the ring and is controlled by a controller. Preferably, the ring is disposed in an annular collector which is formed with an annular slot opposite the orifices.
To carry the second variant of the process into effect, the ring is disposed in a collector which is formed, opposite the path of the orifices and at the angular pitch of the vanes, with a plurality of distributed apertures of a diameter at least equal to the diameter of the corresponding orifice. The device also comprises means for positioning the orifices in front of a set of apertures in dependence upon engine speed.
Other advantages and features of the invention will become apparent from the following description of the preferred embodiments, given by way of example, and with reference to the accompanying drawings.